Prostate cancer is the most commonly diagnosed malignancy and the second most frequent cause of cancer-related deaths in the western male population. Prostate cancer therapies are most effective in the earlier stages of the disease, before metastasis has occurred. Treatment is expected to be even more effective before significant local growth of the cancerous tissue has taken place. Therefore, efforts to control the disease (i.e., to decrease prostate cancer mortality) have focused on increasing detection of the cancer while it is still locally confined and potentially curable, through diagnostic assays that are suitable for early detection of prostate cancer. Unfortunately, such detection also has significant drawbacks, because diagnostic assays that use currently available prostate cancer markers lead to high numbers of false positive diagnoses, and/or are not sufficiently sensitive (potentially leading to high numbers of false negative diagnoses).
Measurements of serum concentrations of prostatic marker enzymes have recognized value in the clinical detection, diagnosis and management of prostate cancer. The two most widely used prostatic marker enzymes are prostatic acid phosphatase (PAP) and prostate-specific antigen (PSA). Normally, both enzymes are secreted from the prostatic epithelial cells into the seminal fluid, but in patients with prostatic disease they leak into the circulation, where they can be detected by means of immunological assays (Armbruster, Clin. Che. 39:181-95 (1993)).
Prostatic acid phosphatase, one of the earliest serum markers for prostate, has an as yet undetermined function and is one of the most predominant protein components in human prostatic secretions. The use of PAP as a marker for prostatic tumors is complicated by the reported structural similarities between the prostate-specific acid phosphatase and the lysosomal acid phosphatase occurring in all tissues. Furthermore, there is a tendency towards lower PAP mRNA and protein levels in prostate cancer in comparison with benign prostatic hyperplasia (BPH). In recent years, PAP measurements were superseded by serum PSA measurements in the clinical management of prostate cancer.
Prostate-specific antigen (PSA) was identified by several groups as a prostate-specific protein from the seminal fluid, and was subsequently determined to be an antigen from prostate cancer tissue. PSA is produced exclusively by the columnar epithelial cells of the prostate and periuretural glands. Normal prostate epithelium and benign hyperplastic tissue actually produce more PSA mRNA and protein than does prostate cancer tissue. Furthermore, it was shown that loss of differentiation of prostatic carcinomas is associated with a decrease in the level of intraprostatic PSA.
Prostate-specific membrane antigen (PSM) was originally identified using an antibody developed by immunizing mice with the membrane fraction of LNCaP human prostatic adenocarcinoma cells. Like PAP and PSA, PSM can be detected in normal prostate, BPH and prostate cancer and is absent from most other tissues. However, the usefulness of PSM as marker for prostatic cancer has not been fully established.
Other markers have recently been considered. For example, PCA3 DD3 is a new marker from DiagnoCure, which has been described as being useful in a urine-based test (PCA3 itself is described in PCT Application Nos. WO 98/45420 and WO 2000/123550). This marker is apparently only expressed in prostate cancer, and therefore may be used to distinguish between BPH and prostate cancer. However, as described in greater detail below, the sensitivity and accuracy of this marker may be improved when used in combination with one or more additional markers.
Therefore, PSA is recognized as the best available marker for prostate cancer, being useful for screening selected populations of patients with symptoms indicative of prostate cancer and for monitoring patients after therapy, especially after surgical prostatectomy. However, PSA has significant drawbacks in terms of false positive measurements, since it cannot distinguish prostate cancer from BPH. It may also lead to false negative measurements, since de-differentiation of prostate cancerous tissue (which may occur with some types of prostate cancers) also leads to decreased expression of this marker. New markers are currently being developed to overcome this problem, but these markers have their own drawbacks. Clearly, new markers are required.